You only need to replace the actual bearings if they are worn, you can tell if they are worn by whether water was in the center-post or if the spin-tube is worn.

If the bearings are not worn just replace the top seal and relubricate the bearings and seals.

If you need to replace the bearings and do not have the proper bearing installing tool I have heard of people pounding them in place, but I would not try it as you will likley destroy the bearings in the process.